1. Field of the Invention
The invention relates to a headrest control apparatus, a headrest control method and an active headrest which move a headrest to prepare for a rear-end collision with a subsequent vehicle.
2. Description of the Related Art
A headrest apparatus for a vehicle, which includes a headrest that holds a passenger's head and that is fitted to a top end portion of a seat back, is described in, for example, Japanese Patent Application Publication No. 2003-191778 (JP-A-2003-191778). The headrest apparatus for a vehicle described in JP-A-2003-191778 further includes front holding means for moving a head holding portion of the headrest toward the front of a vehicle so that the passenger's head is held at a position closer to the front of the vehicle and therefore prevented from moving rearward in the event of a rear-end collision. Furthermore, the headrest apparatus includes upper-side portion forward tilting means for causing an upper side portion of the head holding portion to travel for a longer distance than the lower side portion thereof does when the head holding portion is moved toward the front of the vehicle.
However, the direction of an inertia force applied to the passenger's head at the time of an actual rear-end collision is not always the rearward direction which is parallel to the longitudinal axial direction of the vehicle. This is because the direction of an inertia force applied to the passenger's head varies depending on the manner in which the host vehicle is rear-ended by the subsequent vehicle, the steering angle of the host vehicle (the vehicle which is rear-ended by the subsequent vehicle), etc. Nevertheless, such variation in the direction of an inertial force applied to the passenger's head is not taken into account in the configuration of the headrest apparatus described in JP-A-2003-191778.